This invention relates to a screw type vacuum pump for evacuating a closed chamber to produce a vacuum therein.
Various types of vacuum pumps, such as an oil-sealed rotary pump, a Roots mechanical booster pump, an ejector pump and a diffusion pump, have been in use to obtain medium and rough vacuum in which pressures are higher than about 10.sup.-4 Torr. The vacuum pumps and vacuum systems of the prior art have had a number of problems.
More particularly, the vacuum pump has a narrow range of operation pressures, and it is impossible for a single vacuum pump to operate a pressure range from 760 to 10.sup.-4 Torr level. The oil-sealed rotary pump is practically the only vacuum pump that is capable of operation with the backing pressure at the atmospheric pressure level, and almost all other vacuum pumps are incapable of operation unless the backing pressure is below 10 Torr. This makes it necessary to use an oil-sealed rotary pump in two stages or to use an oil-sealed rotary pump and another pump, such as a Roots pump as a mechanical booster pump, when one desires to achieve ultimate pressures of 10.sup.-1 to 10.sup.-4 Torr in a semiconductor manufacturing apparatus, such as a CVD (chemical vapor deposition) chamber. FIG. 1 shows one example of the prior art vacuum system in which an oil-sealed rotary pump 2 is used as a main process pump for evacuating the vacuum chamber 1 and a mechanical booster pump 3 is used in combination with the oil-sealed rotary pump 2 to achieve the desired level of pressure. In this example, when the pressure in a vacuum chamber 1 is high, a valve 5 is opened, valves 6 and 7 are closed while the oil-sealed rotary pump 2 is actuated to perform evacuation. Then, the valve 5 is closed and the valves 6, 7 are opened when the pressure in the chamber 1 is reduced to a level of less than 10 Torr in which the mechanical booster pump 3 is capable of operation, so that the evacuation operation can be continued by the oil-sealed rotary pump 2 and the mechanical booster pump 3 operating in series with each other. This type of vacuum system of the prior art suffers the disadvantages that it is complex in construction and high in cost, and that the operation of opening and closing the valves is troublesome.
In an oil-sealed rotary pump, a working chamber thereof is full of oil, so that there is the risk that the back-streaming of oil molecules may reduce the level of a vacuum or contaminate the vacuum system. To avoid this problem, it is necessary to mount an oil-trap 4 between the oil-sealed rotary pump 2 and the vacuum chamber 1 to prevent the molecules of oil from invading the vacuum chamber 1. This makes the construction of the vacuum system still more complex. A CVD apparatus uses a reactive gas, such as a hydride, and the active principle of the gas causes decomposition and deterioration of the oil of the vacuum pump, making it necessary to regularly replace the old oil by a new one. This requires a lot of labor and expenses for effecting maintenance.
An object of this invention is to provide a screw type vacuum pump capable of achieving pressures of 10.sup.-1 to 10.sup.-4 Torr level by a single stage.
Another object is to provide a screw type vacuum pump capable of achieving a medium vacuum with pressures of 10.sup.-2 to 10.sup.-4 Torr by a simple construction.
In accordance with the invention a male rotor and a female rotor, with intermeshing helical lands and grooves, cooperate with each other in casings and provide working chambers which provide a gas compression region in which the volume of the working chambers is reduced as the male and female rotors rotate to perform operations of compressing and discharging the gas and a transfer region in which the volume of the working chambers essentially shows no change even if the male and female rotors rotate, and that the working chambers of the gas compression region and the working chambers of the transfer region constitute pairs of working chambers, each pair of working chambers constituting a pair of proportions with respect to one of a plurality of grooves of the male and female rotors.